Trait-environment Associations Research Articles

Environmental conditions differently shape leaf, seed and seedling trait composition between and within elevations of tropical montane forests

The composition of plant functional traits varies in response to environmental conditions due to processes of community assembly and species sorting. However, there is a lack of understanding of how plant trait composition responds to environmental conditions at different spatial scales and across the plant life cycle. We investigated the trait composition of leaves (specific leaf area), seeds (seed mass) and seedlings (initial seedling height) across elevations and within elevations in relation to soil and light conditions in a tropical montane forest in southern Ecuador. We surveyed traits and communities of adult trees, seeds and seedlings on nine plots at three elevations (1000–3000 m a.s.l.) and calculated community‐weighted mean trait values to analyse trait variation across and within elevations. In addition, we measured two environmental factors (soil C/N ratio and canopy openness) to quantify local‐scale variation in environmental conditions within elevations. We found that community‐weighted means of specific leaf area, seed mass and initial seedling height decreased consistently with increasing elevation. Within elevations, mean trait values of trees, seeds and seedlings responded differently to local‐scale environmental conditions. Specific leaf area decreased with increasing soil C/N ratio, and initial seedling height decreased with increasing canopy openness. Seed mass was associated neither with soil nor with light conditions. Our findings show that broad‐scale and local‐scale processes differently shape the composition of leaf, seed and seedling traits in tropical forests, indicating a scale‐dependence in trait–environment associations. Furthermore, plant traits corresponding to different life stages were related differently to environmental conditions within elevations. Community assembly processes may therefore lead to differences in species sorting at early and late plant life stages.

Strong and weak trait–environment associations in subarctic stream diatoms

Strong and weak trait–environment associations in subarctic stream diatoms

Large-scale factors controlling biological communities in the Iberian Peninsula: an insight into global change effects on river ecosystems

The ongoing global environmental change poses a serious threat to rivers. Comprehensive knowledge of how stressors affect biota is critical for supporting effective management and conservation strategies. We evaluated the major gradients influencing spatial variability of freshwater biodiversity in continental Spain using landscape-scale variables representing climate, land use and land cover (LULC), flow regime, geology, topography, and diatom (n = 117), macroinvertebrate (n = 441), and fish (n = 264) communities surveyed in minimally impacted streams. Redundancy analysis identified the environmental factors significantly contributing to community variability, and specific multivariate analyses (RLQ method) were used to assess trait–environment associations. Environmental variables defined the major community change gradients (e.g., mountain–lowland). Siliceous, steep streams with increased precipitation levels favored stalked diatoms, macroinvertebrates with aquatic passive dissemination, and migrating fish. These traits were replaced by adnate diatoms, small macroinvertebrates, and non-migratory fish in lowland streams with warmer climates, calcareous geology, agriculture, and stable flow regimes. Overall, landscape-scale environmental variables better explained fish than diatom and macroinvertebrate community variability, suggesting that these latter communities might be more related to local-scale characteristics (e.g., microhabitat structure, substrate, and water physicochemistry). The upslope environmental gradient of river networks (e.g., slope, temperature, and LULC changes) was paralleled to the observed taxonomy-based and trait-based spatial variability. This result indicates that global change effects on riverine biodiversity could emerge as longitudinal distribution changes within river networks. Implementing management actions focusing simultaneously on water temperature, hydrological regime conservation (e.g., addressing LULC changes), and river continuity might be the best strategy for mitigating global change effects on river biodiversity.

Keep your cool: Overwintering physiology in response to urbanization in the acorn ant, Temnothorax curvispinosus

Winter presents a challenge for survival, yet temperate ectotherms have remarkable physiological adaptations to cope with low-temperature conditions. Under recent climate change, rather than strictly relaxing pressure on overwintering survival, warmer winters can instead disrupt these low-temperature trait-environment associations, with negative consequences for populations. While there is increasing evidence of physiological adaptation to contemporary warming during the growing season, the effects of winter warming on physiological traits are less clear. To address this knowledge gap, we performed a common garden experiment using relatively warm-adapted versus cold-adapted populations of the acorn ant, Temnothorax curvispinosus, sampled across an urban heat island gradient, to explore the effects of winter conditions on plasticity and evolution of physiological traits. We found no evidence of evolutionary divergence in chill coma recovery nor in metabolic rate at either of two test temperatures (4 and 10 °C). Although we found the expected plastic response of increased metabolic rate under the 10 °C acute test temperature as compared with the 4 °C test temperature, this plastic response, (i.e., the acute thermal sensitivity of metabolic rate), was not different across populations. Surprisingly, we found that winter-acclimated urban ant populations exhibited higher heat tolerance compared with rural ant populations, and that the magnitude of divergence was comparable to that observed among growing-season acclimated ants. Finally, we found no evidence of differences between populations with respect to changes in colony size from the beginning to the end of the overwintering experiment. Together, these findings indicate that despite the evolution of higher heat tolerance that is often accompanied by losses in low-temperature tolerance, urban acorn ants have retained several components of low-temperature physiological performance when assessed under ecologically relevant overwintering conditions. Our study suggests the importance of measuring physiological traits under seasonally-relevant conditions to understand the causes and consequences of evolutionary responses to contemporary warming.

Community‐wide trait adaptation, but not plasticity, explains ant community structure in extreme environments

Abstract Quantifying trait–environment associations can help elucidate the processes underpinning the structure of species assemblages. However, most work has focused on trait variation across rather than within species, meaning that processes operating at the intraspecific levels cannot be detected. Incorporating intraspecific trait variation in community‐wide analyses can provide valuable insights about the role of morphological adaptation and plasticity on species persistence and the composition of ecological communities. Here, we assessed geographical variation in the direction (i.e. adaptation) and strength of selection, and the magnitude of plasticity, by examining community‐wide trait variation in ant communities along an environmental gradient spanning 9° latitude in Quebec, Canada. Specifically, we measured 9 morphological traits related to foraging strategies, resource use and thermal regulation at 20 locations across temperate and boreal forests. We then examined how the mean and variance of these traits varied along temperature and precipitation gradients. Moreover, we examined how these trait–environment relationships varied across levels of organization, from individual workers (intraspecific) to colonies (intraspecific) and species (interspecific). We observed changes in mean trait values along environmental gradients, but very little change in variance. Specifically, we observed a decrease in the mean length of antennae and an increase in the mean eye length from mild (warm and wet) to more extreme environments (cold and dry). These shifts in trait means were mostly coordinated across organizational levels (i.e. worker, colony and species). We also observed a general increase in trait variance from mild to extreme environments, but only at the species level. Our findings suggest that stressful environmental conditions exert a strong selection pressure on ant morphology causing shifts in optimal trait values. These adaptations may enable persistence at the northern edge of the boreal forest and therefore influence the composition of these ant communities. Specifically, ants with large eyes and short antennae are overrepresented at the transition zone between the boreal forest and the tundra, possibly representing an adaptation to these more open habitats. Our study demonstrates that combining spatial and community‐wide intraspecific functional trait data provides a promising way forward to gain new insights on trait adaptations and processes underpinning community structure along environmental gradients. Read the free Plain Language Summary for this article on the Journal blog.

Hay meadows' overriding effect shapes ground beetle functional diversity in mountainous landscapes

AbstractMountain regions are hotspots of biodiversity, and are particularly sensitive to human activities and global changes. Characterizing biodiversity using trait‐based approaches may improve the understanding of the evolutionary and mechanistic basis of ecological patterns in species distribution. The investigation of trait–environment relationships, however, is challenging, especially when a hierarchical sampling design is used, as this may lead to misidentification of associations. Here, we investigate how functional traits in ground beetles (Coleoptera: Carabidae), a taxon often used as a bioindicator of climate and environmental changes, vary with environmental features in a mountainous landscape. The study was conducted in the Stelvio National Park (Central Italian Alps), collecting individuals with pitfall traps deployed within plots (small spatial scale), located along altitudinal transects (large spatial scale). To investigate the trait–environment association, we used double constrained correspondence analysis, which permits the selection of important traits and environmental variables while accounting for the hierarchical structure of the sampling design. The trait–environment association was largely one‐dimensional, with hay meadow acting as main environmental driver, negatively related to brachypterous wing‐form (indicator of poor dispersal ability) and, to a lesser extent, to specialized diet and (only for the large scale) body length. Secondarily, these traits were related negatively to soil pH and, for the larger spatial scale, positively to canopy cover and elevation. Double constrained correspondence analysis with specialized permutation schemes for statistical testing was effective and robust to analyze the data of the hierarchical sampling.

Phenotypes and environment predict seedling survival for seven co-occurring Great Basin plant taxa growing with invasive grass.

Trait–environment correlations can arise from local adaptation and can identify genetically and environmentally appropriate seeds for restoration projects. However, anthropogenic changes can disrupt the relationships between traits and fitness. Finding the best seed sources for restoration may rely on describing plant traits adaptive in disturbed and invaded environments, recognizing that while traits may differ among species and functional groups, there may be similarities in the strategies that increase seedling establishment. Focusing on three grass genera, two shrub species, and two forb genera, we collected seeds of all taxa from 16 common sites in the sagebrush steppe of the western United States. We measured seed and seedling characteristics, including seed size, emergence timing, and root and shoot traits, and compiled a suite of environmental variables for each collection site. We described trait–environment associations and asked how traits or environment of origin were associated with seedling survival in invaded gardens. Sampling seven taxa from the same sites allowed us to ask how trait–environment–performance associations differ among taxa and whether natural selection favors similar traits across multiple taxa and functional groups. All taxa showed trait–environment associations consistent with local adaptation, and both environment of origin and phenotypes predicted survival in competitive restoration settings, with some commonalities among taxa. Notably, rapid emergence and larger seeds increased survival for multiple taxa. Environmental factors at collection sites, including lower slopes (especially for grasses), greater mean annual temperatures (especially for shrubs and forbs), and greater precipitation seasonality were frequently associated with increased survival. We noted one collection site with high seedling survival across all seven taxa, suggesting that conditions within some sites may result in selection for traits that increase establishment for multiple species. Thus, choosing native plant sources with the most adaptive traits, along with matching climates, will likely improve the restoration of invaded communities.

Do morphological traits of ground-dwelling ants respond to land use changes in a neotropical landscape?

Analyzing the impacts of agricultural activities on biodiversity requires a full knowledge of the ecology of the studied species. Using a trait-based approach may improve our ability to understand land use impacts on less well-studied species in order to establish general trends that will facilitate the prediction of these impacts. In this study, we applied a trait-based approach to understand the impact of land use change on ant communities in the Colombian Llanos region. Five common land uses were considered (annual crops, rubber plantations, oil palm plantations, improved pastures and semi-natural savannas) to test whether some morphological traits respond to soil properties and land uses. An RLQ analysis shows a significant common structure between species distribution, environmental factors, and morphological ant traits. This indicates that morphological traits could be used to predict the response of ant communities in different land uses since they respond to environmental characteristics as vegetation complexity, composition and management. Based on the selected morphological ant traits, three groups of land uses were differentiated: grazing-based systems, agroforestry plantations and annual crops. Agroforestry plantations, especially rubber plantations tend to host larger and specialized ant species, while grazing-based systems (i.e. improved pastures) mainly host small and generalist ants, and annual crops host more pigmented ants. These findings suggest that certain morphological traits can reflect the ability of ant species to settle down and survive in a given land use system. Our study shows that improving knowledge about trait-environment associations could be a useful way to better understand how ecological filtering shape neotropical ant communities and how they respond to landscape transformation and land use changes.

Trait-Environment Relationships Reveal the Success of Alien Plants Invasiveness in an Urbanized Landscape.

Urban areas are being affected by rapidly increasing human-made pressures that can strongly homogenize biodiversity, reduce habitat heterogeneity, and facilitate the invasion of alien species. One of the key concerns in invaded urban areas is comparing the trait–environment relationships between alien and native species, to determine the underlying causes of invasiveness. In the current study, we used a trait–environment dataset of 130 native plants and 33 alien plants, recorded in 100 plots covering 50 urban areas and 50 non-urban ones in an urbanization gradient in the arid mountainous Saint-Katherine protected area in Egypt. We measured eleven morphological plant traits for each plant species and ten environmental variables in each plot, including soil resources and human-made pressures, to construct trait–environment associations using a fourth-corner analysis. In addition, we measured the mean functional and phylogenetic distances between the two species groups along an urbanization gradient. Our results revealed strongly significant relationships of alien species traits with human-made pressures and soil resources in urban areas. However, in non-urban areas, alien species traits showed weak and non-significant associations with the environment. Simultaneously, native plants showed consistency in their trait–environment relationships in urban and non-urban areas. In line with these results, the functional and phylogenetic distances declined between the aliens and natives in urban areas, indicating biotic homogenization with increasing urbanization, and increased in non-urban areas, indicating greater divergence between the two species groups. Thereby, this study provided evidence that urbanization can reveal the plasticity of alien species and can also be the leading cause of homogenization in an arid urban area. Future urban studies should investigate the potential causes of taxonomic, genetic, and functional homogenization in species composition in formerly more diverse urbanized areas.

Adaptation of Mediterranean forest species to climate: Lessons from common garden experiments

Abstract Mediterranean ecosystems are biodiversity hotspots located between temperate mesic climates and semi‐deserts and deserts. Mediterranean climates are characterized by a drought season but its length and severity can be highly variable across regions. In this review, we explored population genetic variation in functional traits and fitness in Mediterranean oak and pine species. We tested the hypothesis that increased drought tolerance has evolved as an adaptation to xeric and warm areas of the Mediterranean climate while increased competitive abilities have been favoured in more mesic regions, following a productivity‐persistence trade‐off within species. We first reviewed the literature and performed a meta‐analysis of studies on Pinus sp. and Quercus sp. from Mediterranean climates, and then analysed a set of unpublished data from Spanish common garden experiments of species from the Mediterranean Basin, searching for evidence of trait genetic variation among populations, local adaptation and trait–environment associations. The meta‐analysis showed that 82.9% (95% CI 75.7–89.4) of traits per study showed significant population genetic differentiation and that 52.5% (35.6–69.2) of traits in studies with two or more Mediterranean environments exhibited population‐by‐environment interaction. Population differences in traits related to drought tolerance, cold hardiness and fitness were usually correlated with provenance precipitation and temperature gradients, suggesting that adaptation to different climatic regimes is an important process driving intraspecific genetic divergence. Within the Spanish trial network, however, predicted fitness functions based on climatic transfer distance failed to reveal significantly higher fitness of populations at experimental sites with climates closer to their provenances’ ones. Neither the literature review nor the new analyses of the Spanish trial network provided evidence of a negative association between growth‐ and persistence‐related traits at the intraspecific level. Synthesis. Population genetic differentiation in functional traits and fitness is common in Mediterranean species, driven, at least in part, by the adaptation to contrasting temperature and precipitation regimes. Because of experimental constraints or biological reality, our study failed to detect a trade‐off between resource‐use strategies within species, calling for future studies to investigate further whether intraspecific evolution of stress tolerance occurs at the expense of growth potential in Mediterranean environments or not.

High elevation bird communities in the Swiss Alps exhibit reduced fecundity and lifespan independently of phylogenetic effects

The merging of community ecology and phylogenetic biology allows us to link broader evolutionary processes to local ecological processes, thereby increasing our understanding of community assembly. A recurrent way to test how species assemblages respond to different abiotic conditions and evaluate the role of evolutionary constraints in community assembly is through using environmental gradients as natural treatments. Here, we combine phylogenetic and trait-based methods to evaluate how the phylogenetic diversity and composition of bird assemblages and their community-weighted traits vary along an elevational gradient in the Swiss Alps. For this purpose, we used four life-history traits considered to be key indicators of individual species response to environmental changes: clutch size, number of breeding attempts, dispersal capacity and lifespan. Controlling for phylogeny, we determined whether environmental filters (elevation, habitat type) act on these traits independently of the level of relatedness among species. We found that phylogenetic dispersion decreases with elevation, but the signature of phylogenetic clustering was weak. Phylogenetic fuzzy weighting showed that the distribution of bird species across plots was related to the two environmental gradients; nonetheless, such influence was not determined by the phylogenetic relationships in either case. That is, there are no specific clades associated with particular elevation or habitat types. We also found that high elevation communities around the treeline were composed of species with lower reproductive rates, reduced lifespan, and lower dispersal capacity, which would make them less resilient to environmental change. Although traits showed moderate phylogenetic signal, only the lifespan was phylogenetically structured. In the remaining cases, the trait-environment association was not mediated by the phylogenetic relationships among taxa. Our study indicates that evolutionary constraints do not represent a significant driver of community assembly in Alpine bird communities and support the notion that phylogeny may often not be a good proxy for traits subject to environmental filtering.

Area threshold and trait–environment associations of butterfly assemblages in the Zhoushan Archipelago, China

AbstractAimAn area threshold is usually considered as the point of abrupt change in the species–area relationship (SAR). Despite their widespread existence, few studies have considered the influence of data transformations on their presence or the mechanisms that generate them. The aims of this study were thus to test for the existence of an area threshold in butterfly assemblages and to compare trait–environment associations between small and large islands split by the threshold.LocationZhoushan Archipelago, China.TaxonButterflies.MethodsWe used the line‐transect method to survey butterflies on 42 islands from July to August 2014. We also collected data on three commonly used ecological traits of butterflies (wingspan, wing loading and body weight). We transformed the butterfly datasets into three types (log‐log, semi‐log and untransformed) and fitted each of them using a set of breakpoint regression models. We then split all islands into small and large islands based on the area threshold and analysed the trait–environment associations for each island group.ResultsTwo‐slope breakpoint models performed best for log‐log and semi‐log transformed datasets. However, the former exhibited a steep‐shallow shape with an area threshold of 0.820 km2 and the latter was in shallow‐steep shape with a threshold of 17.022 km2. By contrast, no threshold was detected when using the untransformed power model. Moreover, for small islands, small area and large isolation were positively correlated with species with large wing loading and body weight. For large islands, however, the perimeter to area ratio (PAR) was significantly and positively related to wing loading.Main conclusionsWe found that data transformations may influence the detection of area thresholds. Trait–environment analyses can further provide novel and mechanistic insights into processes generating area thresholds. We urge that the influence of data transformations and ecological traits of species should be explicitly considered in future studies.

Trait-environment relationships differ between mixed-species flocking and nonflocking bird assemblages.

Hypotheses about the mechanisms of community assembly suggest that biotic and abiotic filters constrain species establishment through selection on their functional traits. It is unclear how differences in traits influence the niche dimensions of closely related bird species when they coexist in spatiotemporally heterogeneous environments. Further, it is necessary to take into account their participation in mixed-species flocks, social systems that can include both competition and facilitation. For 6 yr, we conducted counts of forest bird species and took measurements of environmental variables along an elevational gradient in the Nanling Mountains, China. To disentangle different deterministic and historical/stochastic processes between flocking and nonflocking bird assemblages, we first compared phylogenetic and functional structure, and community-weighted mean trait values (CWM). We further assessed elevational variations in trait-environment relationships. We found that the flocking and nonflocking bird assemblages were structured by environmental gradients in contrasting ways. The nonflocking assemblage showed a strong change from over-dispersed to clustered community structure with increasing elevations, consistent with the strong selective pressures of a harsh environment (i.e., environmental filtering). The nonflocking assemblage also displayed significant trait-environment relationships in bivariate correlations and multivariate ordination space, including specific morphological and foraging traits that are linked to vegetation characteristics (e.g., short trees at high elevations). By contrast, flocking birds were more resilient to habitat change with elevation, with relatively consistent community membership, and showed fewer trait-environment associations. CWM of traits that are known to be associated with species' propensity to join mixed-species flocks, including small body size and broad habitat specificity, were linked to the flocking assemblage consistently across the elevational gradient. Collectively, our trait-based analyses provide strong evidence that trait-environment relationships differ between flocking and nonflocking bird assemblages. Besides serving as bellwethers of changing environments, emergent properties of flock systems may increase the resilience of animal communities undergoing environmental change. Mixed-species flocks present an ideal model with which to explore cooccurrence of closely related species, because habitat filtering may be buffered, and the patterns observed are therefore the outcomes of species interactions including both competition and facilitation.

Drivers of Functional Composition of Bird Assemblages in Green Spaces of a Neotropical City: A Case Study From Merida, Mexico

Given current urbanization trends, understanding the factors that affect local biodiversity is paramount for designing sound management practices. Existing evidence suggests that the assembly of urban communities is influenced by the environmental filtering of organisms based on their traits. Here, we investigate how environmental characteristics including isolation measurements affect the functional composition of avian assemblages in green spaces of Merida, Mexico, a Neotropical city. We sampled 22 sites, analyzed point-count data collected during fall migration, and characterized the habitat with regard to floristic and structural vegetation attributes, vegetation cover within green spaces, urban infrastructure, and isolation. We assessed the relationship between habitat descriptors and bird functional traits using RLQ and fourth-corner tests and compared trait–environment associations between resident and wintering species. Our results showed that functional composition of resident bird assemblages was linked to the environmental characteristics of the site, while the functional composition of wintering species was not. In particular, the degree of isolation revealed to be an important determinant of trait composition. Plant species richness, particularly native tree and shrub species, were critical for the functional composition of resident birds in green spaces. Our findings suggested shifts in body mass from less to more isolated green spaces. Specifically, we observed that large-bodied species predominated in isolated green spaces. This information is useful given the predicted increases in habitat isolation and transformation of green spaces due to urbanization.

Functional traits explain the Hutchinsonian niches of plant species

AbstractAimThe Hutchinsonian niche is a foundational concept in ecology and evolutionary biology that describes fundamental characteristics of any species: the global maximum population growth rate (rmax); the niche optimum (the environment for which rmax is reached); and the niche width (the environmental range for which intrinsic population growth rates are positive). We examine whether these characteristics are related to inter‐ and intraspecific variation in functional traits.LocationCape Floristic Region, South Africa.Time periodPresent day.Major taxa studiedTwenty‐six plant species (Proteaceae).MethodsWe measured leaf, plant‐architectural and seed traits across species geographical ranges. We then examined how species‐mean traits are related to demographically derived niche characteristics of rmax, in addition to niche optima and widths in five environmental dimensions, and how intraspecific trait variation is related to niche widths.ResultsInterspecific trait variation generally exceeded range‐wide intraspecific trait variation. Species‐mean trait values were associated with variation in rmax (R2 = 0.27) but were more strongly related to niche optima (mean R2 = 0.56). These relationships generally matched trait–environment associations described in the literature. Both species‐mean traits and intraspecific trait variability were strongly related to niche widths (R2 = 0.66 and 0.59, respectively). Moreover, niche widths increased with intraspecific trait variability. Overall, the different niche characteristics were associated with few, largely non‐overlapping sets of traits.Main conclusionsOur study relating functional traits to Hutchinsonian niches demonstrates that key demographic properties of species relate to few traits with relatively strong effects. Our results further support the hypothesis that intraspecific trait variation increases species niche widths. Given that niche characteristics were related to distinct sets of traits, different aspects of environmental change might affect axes of trait variation independently. Trait‐based studies of Hutchinsonian niches thus yield important insights into the mechanisms shaping functional biodiversity, which should reinforce the role of traits in functional biogeography.

New robust weighted averaging‐ and model‐based methods for assessing trait–environment relationships

Abstract Statistical analysis of trait–environment association is challenging owing to the lack of a common observation unit: Community‐weighted mean regression (CWMr) uses site points, multilevel models focus on species points, and the fourth‐corner correlation uses all species‐site combinations. This situation invites the development of new methods capable of using all observation levels. To this end, new multilevel and weighted averaging‐based regression methods are proposed. Compared to existing methods, the new multilevel method, called MLM3, has additional site‐related random effects; they represent the unknowns in the environment that interact with the trait. The new weighted averaging method combines site‐level CWMr with a species‐level regression of Species Niche Centroids on to the trait. Because species can vary enormously in frequency and abundance giving diversity variation among sites, the regressions are weighted by Hill's effective number (N2) of occurrences of each species and the N2‐diversity of a site, and are subsequently combined in a sequential test procedure known as the max test. Using the test statistics of these new methods, the permutation‐based max test provides strong statistical evidence for trait–environment association in a plant community dataset, where existing methods show weak evidence. In simulations, the existing multilevel model showed bias and type I error inflation, whereas MLM3 did not. Out of the weighted averaging‐based regression methods, the N2‐weighted version best controlled the type I error rate. MLM3 was superior to the weighted averaging‐based methods with up to 30% more power. Both methods can be extended (a) to account for phylogeny and spatial autocorrelation and (b) to select functional traits and environmental variables from a greater set of variables.

Microscale trait-environment associations in two closely-related South African shrubs.

Plant traits are often associated with the environments in which they occur, but these associations often differ across spatial and phylogenetic scales. Here we study the relationship between microenvironment, microgeographical location, and traits within populations using co-occurring populations of two closely related evergreen shrubs in the genus Protea. We measured a suite of functional traits on 147 plants along a single steep mountainside where both species occur, and we used data-loggers and soil analyses to characterize the environment at 10 microsites spanning the elevational gradient. We used Bayesian path analyses to detect trait-environment relationships in the field for each species. We used complementary data from greenhouse grown seedlings derived from wild collected seed to determine whether associations detected in the field are the result of genetic differentiation. Microenvironmental variables differed substantially across our study site. We found strong evidence for six trait-environment associations, although these differed between species. We were unable to detect similar associations in greenhouse-grown seedlings. Several leaf traits were associated with temperature and soil variation in the field, but the inability to detect these in the greenhouse suggests that differences in the field are not the result of genetic differentiation.

Functional traits and community composition: A comparison among community‐weighted means, weighted correlations, and multilevel models

Abstract Of the several approaches that are used to analyse functional trait–environment relationships, the most popular is community‐weighted mean regressions (CWMr) in which species trait values are averaged at the site level and then regressed against environmental variables. Other approaches include model‐based methods and weighted correlations of different metrics of trait–environment associations, the best known of which is the fourth‐corner correlation method. We investigated these three general statistical approaches for trait–environment associations: CWMr, five weighted correlation metrics (Peres‐Neto, Dray, & ter Braak, Ecography, 40, 806–816, 2017), and two multilevel models (MLM) using four different methods for computing p‐values. We first compared the methods applied to a plant community dataset. To determine the validity of the statistical conclusions, we then performed a simulation study. CWMr gave highly significant associations for both traits, whereas the other methods gave a mix of support. CWMr had inflated type I errors for some simulation scenarios, implying that the significant results for the data could be spurious. The weighted correlation methods had generally good type I error control but had low power. One of the multilevel models, that from Jamil, Ozinga, Kleyer, and ter Braak (Journal of Vegetation Science, 24, 988–1000, 2013) had both good type I error control and high power when an appropriate method was used to obtain p‐values. In particular, if there was no correlation among species in their abundances among sites, a parametric bootstrap likelihood ratio test (LRT) gave the best power. When there was correlation among species in their abundances, a conditional parametric LRT had correct type I errors but had lower power. There is no overall best method for identifying trait–environment associations. For the simple task of testing associations between single environmental variables and single traits, the weighted correlations with permutation tests all had good type I error control, and their ease of implementation is an advantage. For the more complex task of multivariate analyses and model fitting, and when high statistical power is needed, we recommend MLM2 (Jamil et al., 2013). However, care must be taken to ensure against inflated type I errors for both weighted correlations and MLM2. Because CWMr exhibited highly inflated type I error rates, it should always be avoided.

Simple parametric tests for trait–environment association

AbstractQuestionThe CWM approach is an easy way of analysing trait–environment association by regressing (or correlating) the mean trait per plot against an environmental variable and assessing the statistical significance of the slope or the associated correlation coefficient. However, the CWM approach does not yield valid tests, as random traits (or random indicator values) are far too often judged significantly related to the environmental variable, even when the trait and environmental variable are extrinsic to (not derived from) the community data. Existing solutions are the ZS‐modified test (Zelený & Schaffers,) and the max (or sequential) test based on the fourth‐corner correlation. Both tests are based on permutations which become cumbersome when many tests need to be carried out and many permutations are required, as in methods that correct for multiple testing. The main goal of this study was to compare these existing permutation‐based solutions and to develop a quick and easy parametric test that can replace them.MethodsThis study decomposes the fourth‐corner correlation in two ways, which suggests a simple parametric approach consisting of assessing the significances of two linear regressions, one plot‐level test as in the CWM approach and one species‐level test, the reverse of the CWM approach, that regresses the environmental mean per species (i.e. the species niche centroid) on to the trait. The tests are combined by taking the maximum p‐value. The type I error rates and power of this parametric max test are examined by simulation of one‐ and two‐dimensional Gaussian models and log‐linear models.ResultsThe ZS‐modified test and the fourth‐corner max test are conservative in different scenarios, the ZS‐modified test being even more conservative than the fourth‐corner. The new parametric max test is shown to control the type I error and has equal or even higher power than permutation tests based on the fourth‐corner, the ZS‐modified test and variants thereof. A weighted version of the new test showed inflated type I error.ConclusionThe combination of two simple regressions is a good alternative to the fourth‐corner and the ZS‐modified test. This combination is also applicable when multiple trait measurements are made per plot.

Genetic differentiation in life history traits and thermal stress performance across a heterogeneous dune landscape in Arabidopsis lyrata.

Over very short spatial scales, the habitat of a species can differ in multiple abiotic and biotic factors. These factors may impose natural selection on several traits and can cause genetic differentiation within a population. We studied multivariate genetic differentiation in a plant species of a sand dune landscape by linking environmental variation with differences in genotypic trait values and gene expression levels to find traits and candidate genes of microgeographical adaptation. Maternal seed families of Arabidopsis lyrata were collected in Saugatuck Dunes State Park, Michigan, USA, and environmental parameters were recorded at each collection site. Offspring plants were raised in climate chambers and exposed to one of three temperature treatments: regular occurrence of frost, heat, or constant control conditions. Several traits were assessed: plant growth, time to flowering, and frost and heat resistance. The strongest trait-environment association was between a fast switch to sexual reproduction and weaker growth under frost, and growing in the open, away from trees. The second strongest association was between the trait combination of small plant size and early flowering under control conditions combined with large size under frost, and the combination of environmental conditions of growing close to trees, at low vegetation cover, on dune bottoms. Gene expression analysis by RNA-seq revealed candidate genes involved in multivariate trait differentiation. The results support the hypothesis that in natural populations, many environmental factors impose selection, and that they affect multiple traits, with the relative direction of trait change being complex. The results highlight that heterogeneity in the selection environment over small spatial scales is a main driver of the maintenance of adaptive genetic variation within populations.